1. Field of the Invention
The present invention pertains to fluid storage tanks. More particularly, the present invention pertains to fluid storage tanks of the type having a floating roof. Specifically, the present invention pertains to a floating roof storage tank in which the floating roof may be supported at more than one elevation above the bottom of the tank, independently of fluid in the tank.
2. Description of the Prior Art
Many types of fluid are stored in tanks having a bottom, vertical side walls and a roof. Many tank roofs are fixed. However, many other tanks are provided with a floating roof which covers the stored fluid but which rides up and down on the fluid surface therein upon changes in the volume of fluids within the tank. Accordingly, vapor space within the tank is kept at a minimum. This reduces the loss of fluids from evaporation and reduces the hazards associated with vapors, particularly hydrocarbon vapors.
The floating roof of a floating roof tank is typically constructed of metal plates welded together to form a circular deck with a rim around its edge to prevent the fluids stored therein from escaping to the top of the floating roof. Most, if not all, floating roof tanks are provided with some type of roof supports which allows the floating roof to be supported at some elevation above the bottom of the tank when there is little or no fluid therein. There is normally a minimum elevation at which the roof should be supported to prevent it from being positioned below tank inlets or outlets since allowing the roof to do so would result in the roof's sinking if additional fluid entered the tank through the inlets or outlets. At other times, it is desirable to support the floating roof at an even higher elevation to provide enough room below the floating roof for performing cleaning, painting or other service operations thereon. This is to allow personnel and equipment easy access and more desirable working conditions under the roof when performing such operations. In addition, work may be carried out in a safe manner
Typically, storage tanks with conventional floating roofs are constructed with support legs which support the floating roof at a predetermined elevation from the bottom of the tank. Due to the desire to be able to support the roof at some higher position above the tank bottom, floating roof tanks have also been constructed in which support legs, made of pipe, are attached to the roof and extend both downwardly and upwardly therefrom for sliding engagement with a second smaller diameter pipe. Pins are used to pin the two pipes so that the roof may be supported at different elevations thereon. Of course, this results in several openings through the roof. These openings do not leak liquids due to the sleeve acting as a well. However, such openings do contribute to vapor loss and subject personnel repositioning the legs at different elevations to exposure from vapors and products stored in the tank.
Additional developments have been made in floating roof tanks in which the floating roof may be supported from a fixed roof by support means which are activatable through access openings in the fixed roof. Examples of such may be seen in U.S. Pat. Nos. 3,815,775 and 3,831,800. However, these types of supports require a fixed roof, access therethrough and personnel on top of the tank for operation thereof.